Tetrabromobisphenol-A is one of the most widely used brominated flame retardants in the world. It is used extensively to provide flame retardation for styrenic thermoplastics and for some thermoset resins.
Processes used for producing tetrabromobisphenol-A generally fall into three categories. The first category includes processes in which substantial amounts of methyl bromide are co-produced along with the tetrabromobisphenol-A. Generally, up to 40-50 pounds of methyl bromide can be expected per 100 pounds of tetrabromobisphenol-A produced. In most cases, the processes within this first category feature reacting bisphenol-A and bromine in methanol. The ar-bromination of the bisphenol-A is a substitution reaction which generates one mole of HBr per ar-bromination site. Thus, for the production of tetrabromobisphenol-A, four moles of HBr are generated per mole of tetrabromobisphenol-A produced. The HBr in turn reacts with the methanol solvent to produce the methyl bromide co-product. After the bisphenol-A and bromine feed are finished, the reactor contents are cooked for one to two hours to complete the reaction. At the end of the reaction, water is added to the reactor contents to precipitate out the desired tetrabromobisphenol-A product.
The second category of processes features the production of tetrabromobisphenol-A without the co-production of substantial amounts of methyl bromide and without the use of oxidants to convert the HBr to Br.sub.2. See for example U.S. Pat. No. 4,990,321; U.S. Pat. No. 5,008,469; U.S. Pat. No. 5,059,726; and U.S. Pat. No. 5,138,103. Generally, these processes brominate the bisphenol-A at a low temperature, e.g., 0 to 20.degree. C., in the presence of a methanol solvent and a specified amount of water. The water and low temperature attenuate the production of methyl bromide by slowing the reaction between methanol and HBr. The amount of water used, however, is not so large as to cause the precipitation of the tetrabromobisphenol-A from the reaction mass. Additional water for that purpose is added at the end of the reaction. This type of process typically uses a fairly long aging or cook period after the reactants have all been fed, and requires additional process time for the final precipitation of tetrabromobisphenol-A via the last water addition.
In the third category are those processes which feature the bromination of bisphenol-A with bromine in the presence of a solvent and, optionally, an oxidant, e.g., H.sub.2 O.sub.2, Cl.sub.2, etc. See for example U.S. Pat. No. 3,929,907; U.S. Pat. No. 4,180,684; U.S. Pat. No. 5,068,463 and Japanese 77/034620 B4 77/09/05. The solvent is generally a water-immiscible halogenated organic compound. Water is used in the reaction mass to provide a two-phase system. As the bisphenol-A is brominated, the tetrabromobisphenol-A is found in the solvent. The co-produced HBr is present in the water. When used, the oxidant oxidizes the HBr to Br.sub.2, which in turn is then available to brominate more bisphenol-A and its underbrominated species. By oxidizing the HBr to Br.sub.2, only about two moles of Br.sub.2 feed are needed per mole of bisphenol-A fed to the reactor. To recover the tetrabromobisphenol-A from the solvent, the solution is cooled until tetrabromobisphenol-A precipitation occurs. The cooling of the solution to recover tetrabromobisphenol-A entails additional expense and process time.
Other problems can be encountered in the development of new technology for tetrabromobisphenol-A production. During bromination, problems may arise because of excessive cleavage of bisphenol-A and/or the several brominated analogs of bisphenol-A. Besides loss of bisphenol-A raw material, such cleavage typically results in loss of bromine due to consequent coproduction of brominated phenols such as tribromophenol. Discoloration of tetrabromobisphenol-A due to presence of colored species in the product is another potential problem that can be encountered. And as is often the case in commercial processing, the formation of significant amounts of undesirable waste products and consequent expense of suitable waste product disposal is still another area of concern.